1. Field of the Invention
The present invention relates to a projection image generation apparatus and method for generating a projection image based on three-dimensional volume data and a program for the apparatus and the method.
2. Description of the Related Art
Conventionally, a medical image display apparatus that has a mode for observing the inner surface of a tubular hollow organ (a tubular cavity or a lumen) of a subject to examine a diseased region (diseased tissue) of the tubular hollow organ, such as blood vessels, intestines, bronchi and arteries, has been known. This mode imitates the function of an endoscope and the inner surface of the tubular hollow organ can be observed as if a user is viewing the inner surface from the inside of the lumen.
In most of such medical image display apparatuses, projection images are generated based on three-dimensional volume data. However, it has been impossible to observe a temporal change in the diseased region of the tubular hollow organ. Therefore, a medical image display apparatus that can be used to observe a change with the passage of time and to observe the tubular hollow organ from various viewpoints has been proposed. In the medical image display apparatus, four-dimensional data that have time information are used. The four-dimensional data include a plurality of sets of three-dimensional volume data that have been obtained by imaging a subject at time intervals Δt (for example, Japanese Unexamined Patent Publication No. 2006-087573).
However, if perspective projection is constantly performed from the same viewpoint with respect to volume data of different temporal phases, the viewpoint, from which the surface of an organ is projected, may become submerged in the organ (positioned within the organ) in some cases, because the organ is moving. Then, it becomes impossible to project the same position of the organ.